I'm not sure if language choice effects need to unit test. I think that good unit tests test a lot more than type errors and although they are related to design patterns, in that design patterns allow easily unit testable code, and having unit tests in place allows easy refactoring.. unit tests would be helpful even if no standard design patterns were in use.
One main barrier to Unit testing that I have personally seen is that a lot of my work is GUI based or simple database CRUD applications. I haven't found a good useful way to unit test those.
And I'd say an absence of errors does not mean no errors exist. While I don't advocate unit testing as a cure to defects it is comforting to make changes on a system and know you don't break something because of coverage.
Of course the flipside of that is that Java's design patterns aren't nearly as important in Ruby/Python as they have functional programming capabilities out-of-the box. Swings and roundabouts.